1. Field of the Invention
This invention relates to hydrocarbon conversion catalysts such as those utilized to catalyze the reaction of hydrogen with organo-sulfur compounds. More particularly, this invention is directed to catalysts useful for the hydrodesulfurization of hydrocarbons, a method for preparing such catalysts, and most particularly the process for utilizing such catalysts for desulfurizing a hydrocarbon gas oil. The invention is especially directed to catalysts of high overall desulfurization activity and stability.
2. Description of the Prior Art
In the refining of hydrocarbons, it is often necessary to convert a hydrocarbon fraction to different forms. Typically, particulate catalysts are utilized to promote desulfurization or denitrogenation reactions when feedstocks such as vacuum gas oils are contacted with such catalysts under conditions of elevated temperature and pressure and in the presence of hydrogen so that the sulfur components are converted to hydrogen sulfide, and the nitrogen components to ammonia.
Conversions of some hydrocarbons are carried out with a catalyst containing Group VIB and Group VIII metals and phosphorus on a refractory oxide support. Compositions containing these and other elements have been previously investigated. For example, catalysts comprising a Group VIB metal, particularly molybdenum or tungsten, a Group VIII metal, particularly cobalt or nickel, and phosphorus on an alumina base have been disclosed in U.S. Pat. Nos. 3,755,196 and 3,840,472. Such catalysts are very often prepared by impregnation, that is, the deposition of the active components on the support base by contact thereof with an aqueous solution containing the active components in dissolved form. U.S. Pat. No. 3,755,196, for example, describes impregnating media and methods for preparing catalysts using stabilized impregnating solutions containing molybdenum plus nickel or cobalt salts with phosphoric acid dissolved in an aqueous medium. U.S. Pat. No. 3,840,472 discloses another process for preparing a stable impregnating solution that includes dissolving molybdenum oxide with an acid of phosphorus followed by subsequent dissolution of a nickel or cobalt compound.
In addition to active metal components, it has been recognized that hydrocarbon conversion catalysts of specific pore size characteristics have proven effective for catalytic processing. For example, a catalyst employed to remove metals from a metals-containing hydrocarbon residua feedstock ordinarily contains a sizeable number of pores of diameter greater than about 100 angstroms. Such a catalyst often exhibits high demetallation activity, but its useful life is often shortened due to pore plugging as a result of contaminant metal deposition on the surface of the catalyst. Conversely, many catalysts exhibiting a suitable degree of desulfurization activity tend to have a sizeable number of pores having a diameter less than about 100 angstroms. Since it is well known that coke is produced during the conversion of sulfur-containing hydrocarbon molecules to hydrogen sulfide, the useful life of such desulfurization catalysts, at least in part, is shortened by the deposition of coke over the active catalytic sites in the pores of the catalysts.
In the catalytic hydroprocessing of heavy hydrocarbon fractions, such as a residuum, a catalyst employed to promote the removal of sulfur is also known to contact and accumulate contaminant metals. Such a catalyst deactivates by the combination of the two above-mentioned ways, i.e., (1) due to the covering of the active catalytic sites on its surfaces by both coke and metals deposition, and (2) due to pore plugging resulting from contaminant metals deposition. On the other hand, during the hydroprocessing of somewhat lighter, relatively metal-free hydrocarbon fractions, such as gas oils and vacuum gas oils, a catalyst employed to promote the removal of sulfur will not sustain metals deactivation. The useful life of such a catalyst is dependent in large part upon coke deposition.
Although conventional catalysts, including those containing both large and small pores, are somewhat active and stable for hydrocarbon conversion reactions, catalysts of yet higher activities and stabilities are still being sought, and especially those employed for sulfur conversion reactions in hydrocarbon gas oil processing wherein coke deposition on the catalyst is desirably minimized. Increasing the activity of a catalyst increases the rate at which a chemical reaction proceeds under given conditions, and increasing the stability of a catalyst increases its resistance to deactivation, that is, the useful life of the catalyst is extended. In general, as the activity of a catalyst is increased, the conditions required to produce a given end product, such as a hydrocarbon of given sulfur content, become more mild. Milder conditions require less energy to achieve the desired product, and catalyst life is extended due to lower coke formation.
It is generally accepted that greater active component uniformity in the catalytic particles improves activity. The formation of a more evenly distributed layer of the active components, such as the metals and their oxides, or sulfides, in sufficient concentration dispersed throughout the surface area of a catalytic support provides for more efficient utilization of the catalytic surface.
Presently, a need exists for a catalyst, especially for desulfurization of hydrocarbon gas oil, having a suitable pore size distribution that provides for a relatively large percentage of surface area in pore sizes of desired size. A further need exists for a highly active desulfurization catalyst with an extended useful life when employed to promote hydrocarbon conversion reactions, particularly hydrodesulfurization of gas oils.
Accordingly, it is an object of the present invention to provide a hydrocarbon conversion catalyst that is highly active and still has a long useful life when employed in a process to promote the desulfurization of a hydrocarbon oil, particularly with respect to the process for removing sulfur compounds from a gas oil using hydrogen.
It is another object to provide a hydrocarbon conversion catalyst having a relatively evenly distributed layer of active components on a support having a narrow pore size distribution.
A further object of the present invention is to provide a catalyst having a controlled amount of surface area in pores of particular sizes.
It is still another object of the invention to provide a method for producing hydrocarbon conversion catalysts of high desulfurization activity and improved stability in comparison to conventional catalysts of similar composition.
Another object of the invention is to provide a desulfurization catalyst produced by a method that includes the impregnation of a refractory oxide support having a narrow pore size distribution with a stable impregnant solution containing a relatively high concentration of dissolved active components.
These and other objects and advantages of the invention will become apparent from the following description.